The impact of NAD(P)H oxidase on the regulation of renal hemodynamics

Endogenous autocrine and paracrine factors which regulate blood flow through the kidney which may be exerted to different degrees in the cortex and medulla. Superoxide anions, generated by enzymes such as NAD (P) H oxidase, have the potential of modulating the tone of the renal microvasculature but their actions are normally limited as a result of scavenging by superoxide dismutase. The aim of this study was to investigate the contribution of the superoxide anions in the regulation of cortical and medullary blood perfusions generated by NADPH oxidase. This was done by blocking superoxide dismutase activity in rats where NADPH oxidase intact or blocked. Two groups (n=9) of male Wistar rats (210-292g) were used with one group maintained on Apocynin (454gm/l) in drinking water for 7-10 days. Anesthesia was induced with 1ml ip chloralose/urethane (16.5/250mg/ml). The right femoral vein was cannulated for infusion of saline (154mM NaCl) at 3ml/h and supplemental doses of anesthetic. The right femoral artery was cannulated for measurement of blood pressure (BP) and Heart rate (HR). Through a flank incision, the left kidney was mobilized and supported in a cup holder to eliminate respiratory movement and a small cannula was inserted 4.5mm into the kidney for intramedullary (i-m) infusion of saline or drugs at 0.6-1.0 ml/h. Two Laser-Doppler microprobes (each 0.5 mm diameter) were inserted 1.5 and 4.0 mm into the kidney to measure cortical and medullary blood perfusion, respectively (100 perfusion units (PU) = 1 V). After 90min, baseline measurements were taken, and then diethyl-dithio-carbamate (DETC) a SOD inhibitor was infused i-m, at 1ml/hr for 30 min at increasing doses of 1, 2 and 4 mg/kg/min. Data, means ± SE, were analyzed using a one-way ANOVA test with significance taken at P<0.05. The apocynin group had a significantly (p<0.05) lower baseline level of BP (93 ± 5 vs115± 5 mmHg), while MBP, but not CBP, had a trend of increase compared to the normal rats. Neither MAP nor HR was affected by the DETC infusions in either group. Infusion of DETC into the normal group caused a reduction in CBP of some 22±3% only at 4mg/kg/min (P< 0.05), whereas there were dose related decreases in MBP reaching some 39±7% at the highest dose (p< 0.01). In the apocynin treated rats, DETC infusion had no effect on CBP at any dose while it caused a dose related decrease in MBP reaching some 27±6% at 4mg/kg/min which was significantly (p< 0.05) less than the response achieved in the normal rats at this dose. The findings demonstrate that superoxide anions normally have a greater role in the regulation of medullary than cortical hemodynamics. They also demonstrate that even following blockade of NADPH oxidase activity, superoxide anions are still generated, particularly in the medulla and most probably from some other cytoplasmic oxidase.